Although genetic recombination is involved in several processes fundamental to the proper functioning of all cells, little is known about the pathways, mechanisms or enzymes involved in these functions in mammalian cells. In this study, I propose to use an extensively characterized, small DNA virus, simian virus 40 (SV40), and SV40-based vectors, to investigate different recombination pathways in mammalian cells. The limited genetic information available in the viral genome makes it quite likely that the host cell enzymes mediate recombination involving viral and/or vector sequences. Moreover, SV40 DNA exists in cells as a complex with proteins (like histones) that are usually also found associated with cellular DNA. Therefore, the virus serves as a good model system for recombination events involving chromatin. I intend to use recombinant DNA methods and dominant selectable marker genes expressed from SV40-based vectors to detect and quantitate recombination in mammalian cells. These assays will be used to answer specific questions concerning the properties and mechanism of homologous and non-homologous recombination. Assays will also be devised for gene conversion, since this mode of recombination has been implicated in the maintenance of sequence homogeneity in multi-gene families and also in gene evolution. These events will be studied in a variety of cell types, including those from patients believed to be aberrant in DNA recombination and/or repair. I also hope to investigate the enzymology of recombination. These studies are intended to provide insights into the mechanisms of exchange, rearrangement and amplification of DNA sequences. They will also provide the basis for designing new mammalian cell vectors capable of specific integration at defined sites in the cellular genome.